A conventional step motor is constructed so that a rotating shaft is supported radially by a conventional supporting element such as a ball bearing. Thus, the conventional step motor is often subjected to problems such as friction and wear, and is essentially equipped with a lubricating device. Accordingly, those skilled in the art have attempted to develop a bearingless step motor in which the rotating shaft can be supported non-frictionally by a magnetic force without using a mechanical bearing such that the operation of the rotating shaft can be controlled actively.
FIG. 4 schematically shows a conventional bearingless step motor, which is disclosed in Korean Patent Application Publication No. 2002-0067863. The conventional bearingless step motor depicted in such figure is capable of raising a rotating shaft without adding a coil for forming a magnetic bearing. The coils of electromagnets 3 of a stator 1 are disconnected from each other so that the electromagnets 3 are driven individually to thereby raise the rotating shaft by the magnetic forces of the electromagnets 3. The position of the rotating shaft is detected by a sensor. To compensate the position of the rotating shaft, the currents having different intensities should be applied to the respective electromagnets 3 according to the position of a rotor 2, thereby generating the magnetic forces having different intensities at the respective electromagnets 3.
However, the above prior art bearingless step motor is structurally complicated, operationally unreliable, power-consumptive and inappropriate to a compact system. This is because it includes multiple electromagnets for a magnetic rise of a rotor and is inevitably equipped with a controller having a complex structure, sensors and amplifiers for simultaneously controlling the rise and rotation of the rotor. The above prior art bearingless step motor is further disadvantageous since a control current must be supplied for the rise of the rotating shaft even when the rotating shaft is in a stationary (non-rotating) state.